1. Field of the Invention
The present invention relates to a sand mold using foamed sand, a molding method of a sand mold, and a molding die for a sand mold.
2. Description of Related Art
On molding a product having a hollow portion, a collapsible sand core is used. As a molding method of a sand core, a technique using foamed sand obtained such that binder containing water glass is stirred with an aggregate so as to be foamed has been developed (for example, Japanese Patent Application Publication No. 2013-169582 (JP 2013-169582 A), Japanese Patent Application Publication No. 2013-111602 (JP 2013-111602 A)). In the technique, a sand-mold molding device 1 illustrated in FIG. 3 is used. The sand-mold molding device 1 is to mold a sand core (sand mold) for aluminum casting, for example, by hardening foamed sand S, and includes a die 2 having a cavity C for molding the sand core, and a filling device 3 for filling the foamed sand S into the cavity C of the die 2.
The die 2 is to form the cavity C by clamping an upper die and a lower die. The die 2 is provided with a filling opening 5 for communicating the cavity C with a sand tank 12 of the filling device 3. The filling device 3 includes a sand tank 12 for mixing and accumulating the foamed sand S and pressurization mechanism (pressurizing means) 13 for pressurizing the foamed sand S in the sand tank 12. Then, the die 2 is set in the sand tank 12, and the foamed sand S in the sand tank 12 is pressurized by the pressurization mechanism 13, so that the foamed sand S is filled into the cavity C of the die 2 via the filling opening 5. The die 2 is heated to around 150° C. to 300° C., so as to vaporize water in the foamed sand S filled in the cavity C, thereby solidifying the foamed sand S. After that, the die 2 is opened and a sand core molded hereby is taken out therefrom.
As described above, when the foamed sand S is filled into the cavity C of the die 2 so as to perform molding, an internal pressure of the cavity C of the die 2 is increased due to evaporation of water and thermal expansion of air bubble in a heating and hardening process of the foamed sand S. Hereby, water glass and an aggregate are accumulated in an outer layer part of a molded product, so that a minute hardened layer having a necessary strength is formed in a predetermined thickness range from the outer layer part of the molded product. As a result, a vulnerable portion that is fragile due to a low density is formed inside the molded product. Further, when the density is markedly unbalanced, the vulnerable portion may become hollow. When the sand core is used for casting, the strength required for the sand core is secured by the hardened layer, and the vulnerable portion or hollow portion contributes to collapsibility after the casting, thereby making it possible to take out the core from a casting product.